Avoid CNC Machining Marks in Your Parts


During the machining process, when end mills are pitted against solid metal, or even plastics, it’s difficult to maintain tooling precisely adjacent to work surfaces without deflecting. Many articles talk about what machinists can do to avoid CNC machining marks. One example is to adjust milling speeds to balance cutting while maintaining surface finishes. There are also things designer-engineers can do, too. These can significantly improve the surface finish of your machined parts.

Machining Chatter and Harmonics

machining marks examples
Examples of machining marks.

The two biggest surface finish issues that can arise when machining metals are chatter and harmonics. Chatter is the bouncing motion in an end mill as it carves out walls and floors. Essentially, the tool subtly rebounds repeatedly against the material surface you want removed. Even worse are undesirable effects called harmonics. These vibrations get amplified because they interact with other motions happening as we mill your parts. It’s a lot like a wave at the beach that grows larger when it merges with another wave. In the end, either of these choppy motions leads to potentially unacceptable finishes.

Design Tips to Avoid CNC Machining Marks

So, here are some things you can do that will help us improve your part’s surface finish.

  • Always maintain a wall thickness of at least .125 in. with a 5x depth ratio. For example, .125 in. thick walls should be no more than .500 in. deep.
  • Design your parts so we can use standard end mill diameters and lengths. This will shorten the time it takes to get your quote back, and also accelerates production lead times. Special extended reach tooling, in particular, requires that we use slower speeds/feeds.
  • If you need square internal corners, break the design up into multiple components. When you do that, the pocket can be machined easily from the side. This allows us to use a flat/square end mill to form those sharp internal corners.

Designing to Improve End Mill Performance

  • Typically, end mills have optimal performance when their length is about four times their diameter. Some end mills can go up to 10-15 times the diameter. The difficulty is that it requires extended reach tooling to yield a good surface finish.
  • Consider whether a standard length end mill can handle making deep pockets in your designs.
  • Avoid designing deep pockets or inside corners with small radii.

Related Design Tips

Machined Plastics vs. Machined Metals and Molded Plastics
Internal Radius Design Tips
The Advantages of Using myRapid

For more helpful machining tips, download our CNC Machining Design Guide.

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